They argue that under Lucia, the special counsel is really an "employee," not subject to the Appointments Clause:

The Supreme Court's recent decision in Lucia v. SEC explains that if a federal position is only "temporary," then such a position is likely not an "office of the United States." . . . Therefore, [the special counsel] may not be an "officer of the United States" under the rule in Lucia.

As an employee, they argue, the special counsel is subject to the ordinary appointment requirements for any (non-officer) civil servant.

Still, they argue that there are four reasons to question Mueller's appointment, including that he wasn't appointed pursuant to civil-servant rules, that he may exercises outsized power for an employee, and that his for-cause termination protection runs into Justice Scalia's dissent in Morrison. (On that last point, they say: Lucia may afford a potentially soon-to-be-more-conservative Supreme Court the opportunity to do what Judge Brett Kavanaugh speculated about in 2016: make Justice Scalia's Morrisondissent into a majority opinion.")